1. Field of the Invention
The invention relates to an automatic transmission, and more particularly, an automatic transmission provided with a parallel-axis gear pair.
2. Description of the Related Art
An automatic transmission provided with a parallel-axis gear pair has been known. For example, there is an automatic transmission provided with a plurality of planetary gear sets, which has a countershaft provided parallel with an input shaft, and a parallel-axis gear pair for transmitting the rotation of the input shaft to the countershaft. One example of this kind of automatic transmission is disclosed in a non-patent reference, “Manual for the New Model Harrier”, published by Toyota Motor Corporation on Dec. 25, 1997. FIG. 4 shows a cross-section of a portion of the automatic transmission described on pages 2-2 to 2-8 of that reference.
As shown in FIG. 4, a counter drive gear 100 is able to rotate around a second input shaft 104 that rotates together with a first input shaft 102 that is coupled to a torque converter, not shown. A counter driven gear 106 is able to rotate around a countershaft 108.
The counter drive gear 100 includes a shaft portion 110 and a gear portion 112, which is perpendicular to the shaft portion 110 and provided on one end of the shaft portion 110. The shaft portion 110 is supported by a supporting wall 118 of a case 116 via a bearing 114.
Also, an outside peripheral portion 112a of the gear portion 112 of the counter drive gear 100 is wider than an inner peripheral portion 112b of the gear portion 112. Further, the center of the outer peripheral portion 112a in the axial direction is closer to the supporting wall 118 side than is the center of the inner peripheral portion 112b in the axial direction, such that the outer peripheral portion 112a protrudes farther toward the supporting wall 118 than does the inner peripheral portion 112b. A concave portion 120 that accommodates the outer peripheral portion 112a of the counter drive gear 100 is formed in the supporting wall 118.
Further, a clutch 130 is provided on the side of the counter drive gear 100 opposite the supporting wall 118. The clutch 130 includes a plurality of friction plates 122, a piston 124 that presses on the plurality of friction plates 122, a spring 126 that urges the piston 124 away from the friction plates 122, and a hydraulic chamber 128 and the like. When the plurality of friction plates 122 are forced together, an intermediate shaft 132 which encompasses the second input shaft 104 and is coupled to a sun gear of a planetary gear set, not shown, becomes coupled to the first input shaft 102.
Accordingly, by forming the concave portion 120 in the supporting wall 118 and accommodating the outer peripheral portion 112a of the gear portion 112 of the counter drive gear 100 in this concave portion 120, the automatic transmission shown in FIG. 4 can be made shorter in the axial direction.
Not only is it necessary to make the dimensions of the automatic transmission shorter in the axial direction, but there is also a need to reduce the shaft to shaft dimension of the automatic transmission, i.e., the dimension of the automatic transmission in the direction perpendicular to the axis. In order to decrease the shaft to shaft dimension of the automatic transmission shown in FIG. 4, it is conceivable to reduce the diameter of the gear portion 112 of the counter drive gear 100. With this structure, however, the concave portion 120 of the supporting wall 118 is prevented from being moved much to the inside in the radial direction by the bearing 114, so unless position of the concave portion 120 is changed, the diameter of the gear 112 of the counter drive gear 100 can not be reduced. While the diameter of the gear portion 112 could be reduced if the counter drive gear 100 were moved toward the first input shaft 102 side, doing so would simply increase the length of the automatic transmission in the axial direction.